Optical circulators are commonly used in optical communication systems and optical measurement systems. FIG. 1a shows a three-port Polarization Maintenance (xe2x80x9cPMxe2x80x9d) circulator 100 that has ports 1, 2, and 3. Each of ports 1, 2, and 3 can be coupled to a PM fiber. As shown in FIG. 1, a polarized optical signal S1 entering port 1 exits from port 2 as a polarized optical signal S2. A polarized optical signal S2xe2x80x2 entering port 2 exits from port 3 as a polarized optical signal S3.
In one aspect, the invention provides an optical device. The optical device includes a non-reciprocal combination-device and a polarizer. The non-reciprocal combination-device has a principal direction and a reverse principal direction. The non-reciprocal combination-device includes a first birefringent wedge, a second birefringent wedge, and a non-reciprocal rotating element. The polarizer receives a first light signal from the second birefringent wedge traveling in the principal direction and transmits a second light signal to enter the second birefringent wedge in the reverse principal direction. The first birefringent wedge has a first optical axis perpendicular to the principal direction. The second birefringent wedge has a second optical axis perpendicular to the principal direction, and the second optical axis forms a first angle with respect to the first optical axis. The non-reciprocal rotating element is optically coupled between the first and the second birefringent wedge. The non-reciprocal rotating element is designed to rotate the polarization of light passing through the non-reciprocal rotating element by a second angle.
In another aspect, the invention provides a method of using a non-reciprocal combination-device as a three-port PM circulator. The non-reciprocal combination-device has a principal direction and a reverse principal direction. The non-reciprocal combination-device includes a first birefringent wedge, a second birefringent wedge, and a non-reciprocal rotating element. The first birefringent wedge has a first optical axis perpendicular to the principal direction. The second birefringent wedge has a second optical axis perpendicular to the principal direction, and the second optical axis forms a first angle with respect to the first optical axis. The non-reciprocal rotating element is optically coupled between the first and the second birefringent wedge. The non-reciprocal rotating element is designed to rotate the polarization of light passing through the non-reciprocal rotating element by a second angle. The method includes the step of providing a first, a second, and a third PM fiber. The method includes the step of directing a first light signal exiting from the first PM fiber with a polarization parallel to the first optical axis to enter the first birefringent wedge in an input direction. The method includes the step of directing the first light signal exiting from the second birefringent wedge with a polarization perpendicular to the second optical axis to enter the second PM fiber. The method includes the step of directing a second light signal exiting from the second PM fiber with a polarization perpendicular to the second optical axis to enter the second birefringent wedge in the reverse principal direction. The method includes the step of directing the second light signal exiting from the first birefringent wedge with a polarization perpendicular to the first optical axis to enter the third PM fiber.
In another aspect, the invention provides a method of using a non-reciprocal combination-device as a three-port PM circulator. The non-reciprocal combination-device has a principal direction and a reverse principal direction. The non-reciprocal combination-device includes a first birefringent wedge, a second birefringent wedge, and a non-reciprocal rotating element. The first birefringent wedge has a first optical axis perpendicular to the principal direction. The second birefringent wedge has a second optical axis perpendicular to the principal direction, and the second optical axis forms a first angle with respect to the first optical axis. The non-reciprocal rotating element is optically coupled between the first and the second birefringent wedge. The non-reciprocal rotating element is designed to rotate the polarization of light passing through the non-reciprocal rotating element by a second angle. The method includes the step of providing a first, a second, and a third PM fiber. The method includes the step of directing a first light signal exiting from the first PM fiber with a polarization perpendicular to the first optical axis to enter the first birefringent wedge in an input direction. The method includes the step of directing the first light signal exiting from the second birefringent wedge with a polarization parallel to the second optical axis to enter the second PM fiber. The method includes the step of directing a second light signal exiting from the second PM fiber with a polarization parallel to the second optical axis to enter the second birefringent wedge in the reverse principal direction. The method includes the step of directing the second light signal exiting from the first birefringent wedge with a polarization parallel to the first optical axis to enter the third PM fiber.
Among the advantages of the invention are one or more of the following. Implementations of the invention provide a three-port PM circulator that can have small insertion loss, compact size, and reduced manufacturing cost. The three-port PM circulator can include isolation function. Other advantages will be readily apparent from the attached figures and the description below.
FIG. 1a illustrates a three-port PM circulator 100 including ports 1, 2, and 3.
FIG. 1b illustrates an implementation of a non-reciprocal combination-device.
FIG. 1c illustrates a specific configuration of birefringent wedges and a Faraday rotator of FIG. 1b. 
FIGS. 1d-1f illustrate alternative configurations of the birefringent wedges and the Faraday rotator of FIG. 1b. 
FIG. 2a illustrates the paths traveled by light that enters the non-reciprocal combination-device of FIG. 1b in the principal direction.
FIG. 2b illustrates that light entering the first birefringent wedge as an e-ray in the principal direction exits from the second birefringent wedge as an o-ray in the first output direction.
FIG. 2c illustrates that light entering the first birefringent wedge as an o-ray in the principal direction exits from the second birefringent wedge as an e-ray in the second output direction.
FIG. 3a illustrates the paths traveled by light that enters the non-reciprocal combination-device of FIG. 1b in the first and the second input direction.
FIG. 3b illustrates that light entering the second birefringent wedge as an e-ray in the first input direction exits from the second birefringent wedge as an o-ray in the principal direction.
FIG. 3c illustrates that light entering the first birefringent wedge as an o-ray in the second input direction exits from the second birefringent wedge as an e-ray in the principal direction.
FIG. 4a illustrates the paths traveled by light that enters the non-reciprocal combination-device of FIG. 1b in the reverse principal direction.
FIG. 4b illustrates that light entering the second birefringent wedge as an e-ray in the reverse principal direction exits from the first birefringent wedge as an e-ray in the reverse principal direction.
FIG. 4c illustrates that light entering the second birefringent wedge as an o-ray in the reverse principal direction exits from the first birefringent wedge as an o-ray in,the reverse principal direction.
FIGS. 5a and 5b illustrate that a polarized optical signal S1 exiting from PM fiber 510 enters PM fiber 520 as a polarized optical signal S2.
FIGS. 6a and 6b illustrate that a polarized optical signal S2xe2x80x2 exiting from PM fiber 520 enters PM fiber 530 as a polarized optical signal S3.
FIGS. 7a and 7b illustrate a three-port PM circulator configured as an isolator.
FIGS. 8a and 8b illustrate that a polarized optical signal S1 exiting from PM fiber 510 enters PM fiber 520 as a polarized optical signal S2.
FIGS. 9a and 9b illustrate that a polarized optical signal S2xe2x80x2 exiting from PM fiber 520 enters PM fiber 530 as a polarized optical signal S3.
FIGS. 10a and 10b illustrate a three-port PM circulator configured as an isolator.
FIGS. 11a and 11b illustrate an implementation of three-port PM circulator 100 that includes a polarizer having a polarization direction perpendicular to the optical axis of birefringent wedge 17.
FIGS. 12a and 12b illustrate an implementation of three-port PM circulator 100 that includes a polarizer having a polarization direction parallel to the optical axis of birefringent wedge 17.
FIG. 13a illustrates an implementation of non-reciprocal combination-device 10 constructed using birefringent crystal materials with index ne larger than no.
FIG. 13b illustrates an implementation of non-reciprocal combination-device 10 constructed using birefringent crystal materials with index ne smaller than no.